1. Field of the Invention
The invention concerns an inverter for feeding sinusoidal currents into an ac network or into a public power supply network.
2. Description of the Related Art
In the case of such inverters power switches are almost exclusively in the configuration of a three-phase bridge, as shown in FIG. 1. Such an inverter produces from a dc/voltage source a multi-phase alternating current of the phases U, V and W. By virtue of the anti-parallel connection of the power switches T1 to T6, as shown in FIG. 1, with suitable diodes, a four-quadrant mode of operation is possible and thus such an inverter circuit can also be used in highly versatile manner.
A disadvantage with such an inverter circuit is that extremely high energy flows occur in the case of a cross-shortcircuit of two switches, for example T1 and T2, which usually results in total destruction of the inverter and possibly causes a fire to break out and thus culminates in destruction of all connected parts of the installation. A further disadvantage is that, with the increase in the dc voltage, the respective components must be of ever increasing quality, which is only possible when using very expensive components.
The object of the present invention is to improve the ability to withstand short-circuiting of an inverter and at the same time to avoid the above-described disadvantages and in particular avoid as far as possible the need for expensive individual components.
In accordance with the invention that object is attained by an inverter having the features according to one of claims 1 to 3. Advantageous developments are set forth in the appendant claims.
The invention is based of the realization that only a single circuit or switching unit is to be used for the production of a half-oscillation of a sinusoidal oscillation. Therefore, for producing a positive half-oscillation of a sinusoidal oscillation, a different circuit or switching unit is used, than for producing the negative part of the sinusoidal current. The consequence of this is that the switching units for producing the positive half-oscillation and also the negative half-oscillation of the sinusoidal current are separated from each other and are connected together only by way of the common current tapping, wherein the production of the current in a switching portion cannot involve repercussions in the other switching portion because each switching portion is protected in relation to the other by a switch in the current tapping path.
The division of the sinusoidal output current of the inverter into a positive and a negative half-oscillation affords the possibility of sharing the dc voltage supply to the two switching portions for the negative and positive half-oscillations. Therefore the part of the inverter which produces the positive half-oscillation can be operated with a dc voltage. for example Ud1=660 volts and the switching portion of the inverter which produces the negative half-oscillation of the sinusoidal current can also be operated with a dc voltage, for example Ud2=660 volts. As a total dc voltage that then gives double the individual dc voltage, that is to say 1320 volts. That results in double the output power of the inverter overall, when using components which are only designed for a dc voltage of 660 V.
The output inductances of the individual switching units of the inverter are also acted upon for example during the positive current component only with the partial dc voltage Ud1 and not with the total dc voltage Ud1+Ud2. That also results in a saving in terms of material and cost. By virtue of the production of a half-oscillation of a sinusoidal oscillation with a single switching unit, the switching units for different half-oscillations can also be arranged remote from each other in spatial terms, which overall improves the safety and security of the inverter and all parts of the switching installation and also considerably simplifies arranging it in terms of the space involved. A particular advantage of the inverter design according to the invention is that the inductance of the output choke and thus the component costs required for that purpose can be halved.